Don't scroll up when clicking on the same room over and over

If you try to click on your current room in the list, it scrolls up the
messages a bit. This is because in RoomManager::visitRoom it's being
called with an empty eventId and we will happily emit a goToEvent. This
is despite there being nothing to go to.

Fixes #677.

(cherry picked from commit 2379e3d83b)

src/roommanager.cpp

@@ -315,7 +315,9 @@ void RoomManager::visitRoom(Room *r, const QString &eventId)
 
     // It's important that we compare room *objects* here, not just room *ids*, since we need to deal with the object changing when going invite -> joined
     if (m_currentRoom && m_currentRoom == room) {
-        Q_EMIT goToEvent(eventId);
+        if (!eventId.isEmpty()) {
+            Q_EMIT goToEvent(eventId);
+        }
     } else {
         setCurrentRoom(room->id());
     }